The present invention relates to systems for controlling the atmosphere within containers and more particularly to systems for controlling the atmosphere in containers used for the storage and transportation of perishable produce such as fruit, vegetables and flowers.
It is known that during the storage and/or transportation of perishable produce its preservation can be enhanced by controlling the atmosphere surrounding the produce. For example, it has been found that an atmosphere in which nitrogen predominates is particularly useful for controlling the preservation of perishable products. Furthermore, in addition to controlling the amount of oxygen present in the atmosphere there has also been found a need to control the amount of other substances which may be present, for example, carbon dioxide and ethylene which might be formed by the produce whilst contained within the container.
In our European Patent Application EP-A-467668 there is described a system for controlling the atmosphere of a container for use in the storage and/or transportation of perishable goods. The system includes adsorption means in the form of a plurality of beds, each bed being designed preferentially to adsorb from the atmosphere one or more of water vapor, carbon dioxide, oxygen and ethylene. Some of the beds are designed to work on a temperature swing adsorption cycle and at least one bed is designed to operate on a pressure swing adsorption/desorption cycle. Although the system described in our earlier application achieves all of the requirements in terms of atmosphere control it remains complicated and somewhat bulky.
As a simpler alternative to such adsorption/desorption, systems incorporating semi-permeable membranes have been employed; known plants for the separation of gas mixtures, for example, air, by such membranes are constructed so as to present a large surface area of membrane to the air to be separated.
For example, such plants may employ a multitude of identical, elongate, hollow fibers which are formed from a suitable semi-permeable membrane and which extend in parallel to one another. The fibers are usually mounted in and typically extend from one end to the other of a pressure vessel. The air to be separated is fed under pressure into a header at or adjacent one end of the fibers so that it flows longitudinally through the fibers. The insides of the fibers are maintained at a higher pressure than that which obtains on the outside of the fibers. The components of the air diffuse through the membranes at different speeds such that as the air passes along the inside of a membrane so a faster permeating component, for example, oxygen, passes more and more to the low pressure side. Accordingly, a nitrogen rich product gas in the order of 99.5% by volume nitrogen may be withdrawn under pressure from the end of the pressure vessel opposite that at which the feed gas is introduced.
Membrane systems are generally used in two configurations to control the amount of oxygen within food transport containers:
a) to separate nitrogen from the atmosphere surrounding the container and adding it to the container to displace oxygen, or PA1 b) to separate nitrogen from the atmosphere within the container and recirculating the separated nitrogen back into the container to displace oxygen.
In both configurations, the remaining, nitrogen-depleted atmosphere is usually vented to environment. Such systems suffer from the fact that the membranes are highly permeable to carbon dioxide (and water vapor), which therefore tends to pass through the system with the non-nitrogenous part of the atmosphere and is then vented; this leads to carbon dioxide- and water vapor-depletion of the atmosphere within the container, which is damaging to certain high quality perishable products. Accordingly, in practice system a) is usually employed on an intermittent basis so that too much carbon dioxide is not lost, but this is relatively crude, inefficient and unable to maintain the accurate control of atmosphere composition which is necessary for high quality food products. System b) is not often employed, because the loss of carbon dioxide and water vapor is much greater than with system a).
It is an aim of the present invention to address the above problems, and to provide a simple and effective system which will achieve independent control of the gases likely to be found in the atmosphere of a container for perishable goods.